It has been known for many years that a circulation control actuator may be used to effect modulation of the flow around an aircraft wing to effect aerodynamic control of the vehicle in flight, as an alternative to conventional aerodynamic control using flap surfaces. In a circulation control actuator flow issuing from a narrow slot is caused to be turned around a curved surface by the Coanda effect. This in turn modifies the circulation around the aerofoil giving rise to aerodynamic force and moment changes similar to those caused by flap deflection, but without the drag penalty associated with a flap. The magnitude of the response is governed by the air mass flow through rate through the slot and the operating condition of the aerofoil.
It has also been proposed to provide a trailing edge circulation control actuator in which two parallel spaced slots are provided to either side of an aft facing curved Coanda surface, the flow being supplied to one or other slot by a control arrangement with the flow being angularly deflected one sense when issuing through one slot and in the opposite sense when issuing through the other. A problem with this arrangement is it requires two plenum chambers, one for each slot, which deepens the wing section and twin valve arrangements which introduces complexity in terms of the control to the plenum chambers. The increased depth of the wing due to the plenum chambers produces a problem in that the amount of base drag is significant when there is no flow exiting through either slot. Also the variation in demand for fluid flow through the slots means that the operation of the device supplying the pressurised air may be compromised as in general such devices operate more efficiently where there is a constant bleed of air.
U.S. Pat. No. 4,682,746 describes an arrangement in which an aerofoil section is provided with first and second Coanda effect circulation control ports with a control port selection means designed to direct the flow from a plenum chamber exclusively to one port of the other. In one arrangement there is a wedge-shaped seal means rotatable between a position in which flow through one slot is blocked and a position in which flow through the other slot is locked. The width of both Coanda circulation ports is fixed throughout operation. These and other similar bang-bang control devices have been postulated.
A problem with the bang-bang control proposals is that they do not provide any proportional control and this is far from preferred where the actuator is to be controlled by a flight control system which requires the ability to adjust the flow proportionally for stable control of the vehicle.